The Collaborative International Dictionary of English v.0.48:

Immortelle \Im`mor*telle"\, n.; pl. Immortelles. [F. See
   Immortal.] (Bot.)
   A plant with a conspicuous, dry, unwithering involucre, as
   the species of Antennaria, Helichrysum, Gomphrena, etc.
   See Everlasting.
   [1913 Webster]